Provided that the card still does the same back at stock speeds, my guess would be that while that OC was initially stable, it wore something out. OC'ing can do that. It can and often does shorten the lifespan of CPUs and GPUs.
Call me a wimp (or a synonymous 5-letter word), but I always back down a step or two from what I find to be my most aggressive and stable OC. Following that method, I've yet to have an OC'd product die on me.